Switchable step attenuators which are intended to provide attenuation at selectively adjustable levels are well-known in the art. Typically, the prior art attenuator devices employ a series of attenuator sections designed to provide attenuation in discrete steps. The attenuator sections are switched into and out of the circuit by switches, either mechanical or electronic. The well-known "decade box" provides an example of a prior art variable attenuator device.
In RF applications, the attenuator requires broad bandwith, e.g., from DC to gigahertz range, and accuracy, .+-.0.1 dB, and the attenuator must be matched to a transmission line characteristic impedance. Typically, RF attenuators employ several T-attenuator or PI-attenuator sections or elements with switching elements to either select or bypass each attenuator section. Double pole, double throw mechanical switches, or relays, for example, are suitable for low-speed operation. High-speed applications may require rapid switching of attenuator sections for long periods of time. Because of the nature and construction of relays and other types of mechanical switches, such switches tend to wear out when utilized in high speed applications.
Electronic switches, such as a PIN diode, can be used to provide high speed switching of broadband RF signals. PIN diodes have low capacitance when in the OFF state and a low resistance in the ON state and can be switched by a current signal. U.S. Pat. No. 4,359,699 issued to Philip R. Horkin on Nov. 16, 1982 provides an example of PIN diodes used in the prior art to construct a linear attenuator.
Stepped attenuators comprised of several PI or T-pad attenuating elements switched into or out of a circuit utilizing PIN diodes are also known. An example of a prior art switched PI attenuator element is shown in FIG. 1. In the attenuator configuration shown FIG. 1, four PIN diodes and three RF chokes are required. Each attenuator element, when in the bypassed condition, has the insertion loss of two PIN diodes. As a result, PIN diode switched attenuators typically have higher insertion losses than mechanically switched stepped attenuators. The isolation provided by each PIN diode is limited which increases the number of PIN diodes required to provide good RF isolation which further increases the insertion loss. Additionally, as shown in FIG. 1, two control lines are required for each attenuator element.